The Delos system was a planetary system. Two of the planets, Ornara and Brekka, were inhabited by humanoid civilizations.
Before 2164, a Federation scout conducted a reconnaissance of the star system and reported its findings to Starfleet Command. Two centuries later in 2364, the starship USS Enterprise-D visited the system to chart the solar activity of the star Delos. (TNG: "Symbiosis")
According to Startrek.com, there were two systems named Delos.  In addition, according to the episode's script, the system's pronunciation was" DELL-ose".